1. Technical Field of the Invention
The present invention pertains to sulphur free prevulcanized rubber latex compositions, and to a method of producing same and articles produced therefrom. More particularly, the present invention pertains to oxygen-cured natural rubber latex materials, methods of manufacturing the materials and for forming same into thin walled articles such as gloves, dental dams, catheters, and penrose surgical drains so that the resulting articles are sufficiently strong and flexible, and yet are not chemically reactive with other substances which may be contacted by the articles during subsequent use, and so that the articles are otherwise completely safe for applications involving contact with humans.
2. Description of Relevant Art
Rubber is a very common compound. It is available in many different compositions and forms such as rubber latex. In conventional processes for forming rubber, or for forming products from rubber, rubber is cured or vulcanized so as to join the molecules within the rubber composition together giving the rubber composition desired strength and toughness. Most typically sulphur and sulphur donating compounds are used as the primary curing or vulcanizing agents.
For example, there are many publications available, including publications by Uniroyal, Vanderbilt and Monsanto and K. Othmer, Encyclopedia Of Chemical Technology, 3rd Ed., V.20, pp. 337-364 describing starting formulas for sulphur-cure natural rubber systems. Typically, these formulas will include sulphur or a partial substitute of sulphur such as TMTD, DIAK and Sulfasan (partial substitutes of sulfur being preferred especially when controlled release of available sulphur is required), a zinc-based cross linking agent such as zinc dithiocarbamate (ZDBC) and zinc diethylcarbamate (ZDEC), cure rate accelerators (primary and secondary) such as excessive sulphur, ZDBC and sodium diethylcarbamate (SDBC), and antioxidants which are mostly phenolic or amine compounds.
The curing and vulcanization of rubber compounds may be done at various stages relative to the formation or shaping of products from the rubber compounds. For example, in the manufacture of latex gloves from latex emulsions, it is conventional to vulcanize the composition after it has been shaped into gloves (post-vulcanization process), or the latex emulsion may be vulcanized before it is shaped into the gloves (pre-vulcanization). Typically, the chemicals used for vulcanizing the rubber compositions are consumed more efficiently in pre-vulcanization processes because vulcanization occurs with respect to a large mass of material over a longer length of time. On the other hand, pre-vulcanized gloves are known to be tacky and soft in comparison to the post-vulcanized gloves, which leads to handling problems.
Although products formed from such sulphur-cured natural rubber exhibits some very good qualities, including strength, toughness, elasticity, etc. and are used very effectively in many applications, there are some areas or applications where compatibility problems arise due to the chemicals incorporated into the rubber which react undesirably with foreign materials contacted by products formed from the rubber during the products' subsequent use. Particularly, the sulphur, sulphur compounds, dithiocarbamate compounds, zinc and its compounds which are present on and/or in the surfaces of products formed from the rubber may react with foreign materials contacted during use of the products, resulting in contamination, fouling, spoilage or discoloration of the rubber products or of the materials contacted therewith (either directly or indirectly).
A specific example of a problem which has arisen due to a chemical reaction between chemicals in a conventional rubber product and a foreign material contacted by the rubber product during use is in the field of latex examination gloves. Such gloves are almost always used by persons in the medical and dental field during any procedure requiring bodily contact with a patient, such use is even required by federal and/or state laws in many instances and such gloves are also being used in many other fields. Throughout the application the term "latex gloves" refers to any type of latex glove including examination gloves, surgical gloves, and all purpose gloves.
A common procedure involved in dental examinations is the taking of an impression of a patient's upper or lower dental arch, or of one or several teeth. Such impressions are formed using either a gel-like or putty-like dental impression material (DIM) in a metal or plastic container which is pressed against the dental structure to obtain a negative impression of the patient's teeth or arch. Upon removal of the impression material and container from the patient's oral cavity, it is allowed to cure and harden. Once hardened, a second material such as a flowable plaster is poured into the hardened, negative impression material to obtain a positive impression of the teeth or arch. One such DIM is vinyl polysiloxane distributed by 3M under the trademark EXPRESS.TM..
The dental community has recently observed a re-occurring problem in forming negative impressions from the DIM caused by chemical reaction(s) with chemicals contained on/in conventional rubber latex gloves. Specifically, after a negative impression has been obtained of the patient's dental structure, portions of the exposed regions of the DIM which had been in contact (direct or indirect) with the gloves often fail to sufficiently cure and harden. Instead, those regions remain uncured and retain their gel or putty-like consistency. This is most undesirable when attempting to subsequently form the positive impression since the uncured regions of the negative impression do not sufficiently retain and transfer the structure and surface details of the patient's teeth or arch which had been previously contacted, to the positive impression material. Regions of the DIM remain uncured because chemicals on/in the latex gloves react with and tie up the platinum base catalyst of the DIM, which catalyst normally assures rapid curing and hardening of the DIM.
Previously, applicant has proposed to overcome such glove contamination problem through post treatment of the manufactured gloves to tie up and/or barricade the offending chemicals on/in the gloves. Particularly, applicant has proposed to tie up the offending chemicals by treating the exterior surfaces of the latex glove with an acidic suspension of metal particles, such as relatively dilute solution of phosphoric acid (0.25-2.0 molar) with finally divided copper and/oriron particles therein. The suspension is applied to the glove exterior in any appropriate manner such as spraying, brushing, wiping, dipping, etc. after which the treated gloves are allowed to dry. Alternatively, or additionally, a water resistant (preferably insoluble) barricade coating was applied to the exterior of the gloves, which was non-toxic and compatible with the latex. Materials suitable for the proposed barrier coating include silicone spray, polyethylene, polypropylene, copolymers of ethylene, polyvinyl chloride, polyesters, copolymers of vinyl chloride, copolymers of vinyl acetate and ethylene, various polyurethanes, polyamides, polyester polyamides, polysiloxanes and various commercially available polymers utilized for hair spray such as octyl acrylamide, acrylate copolymer VA, crotonates, vinyl neodecanoate copolymer, ethyl ester of polyvinyl methacrylate-methyl acrylate copolymer, and methacrylate copolymer.
Although such post treatments of the conventional latex gloves were usually effective for overcoming undesirable chemical reactions between chemicals of the latex gloves and foreign materials such as the DIM contacted by the latex gloves during use, the treated gloves still undesirably reacted with the DIM approximately 10% of the time and this was an unacceptable failure rate. Further, such post treatments have other problems and disadvantages associated therewith, including the additional cost involved with the post treatments, shelf life and reliability of the post treated gloves, undesired reactions of the post treatment materials with other foreign materials subsequently contacted by the gloves, etc.
Another proposed solution to the DIM contamination problem involves chlorine leaching of conventional sulphur cured, natural rubber latex gloves to remove the offending chemicals from at least the surfaces of the gloves. While such leaching may be effective to reduce many instances of the contamination problem, some degree of contamination still occurs at an unacceptable rate such as 10-15% of the time. Additionally, the chlorine leaching also has disadvantages associated therewith, including the fact that the gloves become less soft and flexible, discolored and have a shorter shelf life.
Another problem encountered with conventional sulphur cured, natural rubber latex products are the nitrosamines contained therein. The nitrosamines, which are often generated from the antioxidants used in natural rubber compositions or by other amine compounds used in the rubber latex, are believed to be carcinogenic. Hence it is undesirable to use the conventional compounds having a high nitrosamine content, where products formed from the rubber composition are likely to be contacted by humans, such as with their skin, body fluids, or in their mouths or other body cavities. For this reason, some natural rubber products namely baby pacifier nipples have been formed from substantially nitrosamine-free rubber compositions in which oxygen-donating curing agents such as peroxides are used instead of conventional sulphur and carbamate curing agents.
Such known nitrosamine-free compositions are useful for products such as pacifier nipples because the products have a relatively large wall thickness to give the products sufficient strength and elasticity, and because the increased tackiness of products resulting from the oxygen-donating curing agents is acceptable and even desirable for the products. On the other hand, such nitrosamine-free compositions have not been used for thin-walled rubber latex products such as gloves, dental dams, catheters, penrosed drains, etc. because the resulting thin walled products have insufficient strength characteristics including tear strength and tensile strength, and because the increased tackiness of the products causes significant problems for handling, storing and donning the products. The reduced strength characteristics and increased tackiness of the oxygen cured rubber latex compositions results from a decreased cross linking density of the compositions in comparison to that of conventional sulfur cured compositions.
Still another problem associated with rubber latex gloves pertains to the latex proteins inherently found therein. Latex proteins, which exist naturally in natural rubber latex ,are essential as an emulsifier to the polyisoprene for maintaining the latex state. Unfortunately, the proteins have caused allergic reactions and other serious health problems in humans, and the latex proteins appear to have a relatively progressive effect on humans such that the undesirable reactions increase in severity with increased exposure to the proteins. For example, conventional latex gloves include a powder or donning agent such as corn starch on the surfaces thereof which facilitates removal of the gloves from a mold and facilitates placing gloves on (donning) and removing the gloves from users' hands. Although the powder acts as a layer between the latex glove and the hand wearing the glove, the latex proteins readily pass through the powder to the skin. In light of the health problems associated with exposure to the latex proteins, two alternative treatments for the gloves have been conventionally utilized to significantly reduce or eliminate the possibility that persons wearing the latex gloves will contact the latex proteins therein. Gloves treated according to such additional treatments are known as "powder-free". The first of such additional treatments is chlorination or chlorine leaching of the gloves, which removes some of the proteins from the gloves. Such chlorination treatment actually weakens the gloves because it initiates a deterioration process, but conventional sulfur-cured gloves remain sufficiently strong even after chlorination. The second of such additional processes involves application of a wax coating whereby the surfaces of the conventional powdered gloves are coated with an ingestible, biodegradable wax material. The wax material may be carnauba wax, which is the same as that used on the candy shells of certain candy products such as M&Ms.RTM.. Although such wax coating does not degrade the desired characteristics of the latex glove, it does tend to melt to some extent after being maintained at body temperature over a period of time, and this is undesirable because the melted wax leaves a residue on users' hands after the gloves are removed, which residue is often subsequently transferred to instruments or other articles handled by the users.
The present invention has been developed to overcome the problems and disadvantages of known rubber latex compositions as discussed above, and to fulfill a great need in the art by providing natural rubber latex compositions and products formed therefrom which are essentially free of active chemical compounds (such as sulphur, dithiocarbamates, zinc compounds, etc.) found in conventional sulphur-cured rubber latex products, which are free of other undesirable chemicals such as nitrosamine, and yet have adequate strength, elasticity and other desired characteristics that they can be used in most applications; as well as an efficient, economical process for manufacturing useful thin walled products such as latex gloves, dental dams, catheters and penrose surgical drains from the natural rubber latex compositions.